April 10th marks the 50th anniversary of the worst single loss in the history of the U.S. Navy. The nuclear fast-attack submarine USS Thresher, built at the Portsmouth Naval Shipyard, was lost during a dive test off the coast of Cape Cod in 1963, killing all 129 people on board. Events to mark the disaster, and remember the crew and their survivors, are scheduled for this weekend. Keith Shortall spoke with the daughter of one of the lost crewmen about how the disaster affected her life and her family.

Marcye Philbrook was just two and half years old when her father, First Quartermaster Julius Francis Marullo Jr., died aboard the Thresher (the sub, right, on launch day). She says she remembers the sadness within the family, but was too young to remember much about him.

"He was just this person I heard stories about. And as a kid you just think it's just a big mistake," she says. "And you fantasize as you grow up that, 'Oh, he's going to show up at our doorstep.' "He's been a P.O.W. all this time.' You know? 'He's been someplace in the world where hasn't been able to contact us, and he'll show up some day.' Obviously that just never happened. You just grow up without a father, basically."

And but for a stroke of fate, Philbrook might have also lost her grandfather that day. He was a naval architect at the Portsmouth Shipyard, and had been scheduled to go out on the Thresher sea trials as well.

"And he got a call that morning: He was bumped by a dignitary who wanted to go, and so he lost his place," Philbrook says. "So my mother would have lost her husband and a father had that happened."

The Thresher was the pride of the Navy when it was commissioned in 1961. It could dive deeper and run quieter than any other sub in the fleet. Thresher had already been through initial trials by 1963, but Philbrook says she's learned that the crew had serious reservations about the deep dive tests - right up to the time they boarded.

"My mom tells me about my father and some of the other men from the submarine sitting around the breakfast table that morning, and they kept saying that the ship wasn't ready to go out, it wasn't ready," she recalls. "And I know that they were afraid. Some people, they made sure their wills were done, they took care of some final arrangements before they went, because they had a really bad feeling about it. But they went anyway."

About 220 miles off Cape Cod, it's believed that an interior pipe burst, and seawater was sprayed onto an electrical panel, shorting it out and causing an emergency shutdown of the nuclear reactor. Without power, Thresher sank, and was crushed under the extreme weight of seawater.

"And it must have been horrible," Philbrook says. "But they did their duty, and you know, they're patriots and they were just good men who were doing what they're supposed to do."

Philbrook says her mother moved on - remarried and had more children. But both mother and daughter plan to attend this weekend's 50th anniverary memorial events for the USS Thresher in Porstmouth and Kittery.